Adobe Photoshop Tutorial – Curves explained

One of the finer qualities of Photoshop is that it offers multiple ways to accomplish a similar goal. Some of the most basic abilities of Photoshop are the image adjustments that allow you to alter image brightness and image contrast. There are three general techniques that allow you to do this and they seem to span the gamut of complexity, with brightness/contrast being the simplest, levels being slightly more complex, and curves being the most complex. Today, I wanted to spend some time explaining the most difficult and powerful technique, Curves, which seems to have an air of secrecy around it.

To locate curves, one must navigate towards Image>adjustments>curves or use the curves selection from the layer adjustment map list. After selecting curves you should see a menu that looks like this:

At a quick glance, it looks harmless enough, but let it be known that curves is a very powerful tool in adjusting the brightness and contrast of an image. Focus your attention on the graph and its respective axes. I find it easiest to speak of the graph in terms of a coordinate plot made up of an x and y axis. Think of the horizontal axis (or x-axis) as all those values in the image ranging from black to white. Values are of course, colors devoid of any saturation. Think of the vertical axis (or y axis) as all those values in the image, ranging from black to white, AFTER adjustments to the brightness and contrast have been made to the image. By default, the graph contains a diagonal line that bisects the graph in two. Imagine a point in the middle of that line. That point corresponds to a value that is a 50% gray. To verify that this is true, draw a vertical line down to the x axis from that point and you should see that it is the midpoint between pure black and pure white. Now, draw a horizontal line to the y axis from that same point. Again, the line should intersect the y axis at the midpoint between pure black and pure white. This make sense because we have yet to make any changes to the image. In other words, any point on that line should have an x and y value that is identical because we haven’t actually adjusted anything yet.

There are three basic shapes that one should utilize when making adjustments with curves. Refer to the image below. The first I will teach is the lighten curve. Click the midpoint of the line and drag up. To understand what we have just done, imagine the coordinates of the new point. The x axis which represents the values in the image BEFORE any changes is still at a 50% gray, but the y axis, which represents the values in the image AFTER we have altered the curve is closer to white. One can see with the coordinates that I have added below the lighten curve, that the y value is lighter than the x value. Take note though, that it is not just one x value in isolation that was affected by the change. In fact, every point on that line is now different, with those values closest to the 50% gray point having the largest incremental change. The important thing is that ALL values of the image are now lighter.

The second curve is a darken curve. It is the exact opposite of the curve explained above and is created by dragging the line downward. Again, it is important to see that every point on the new line is darker, with those values closest to our point, being most affected.

The third curve is a contrast curve. The definition of contrast is “The difference in brightness between the light and dark areas of a picture, such as a photograph or video image.” In other words, to increase contrast, one must increase the difference between the light values and the dark values. We accomplish this by making an S curve. Drag the right portion of the graph up, thereby making the light values lighter and then drag the left portion of the graph down, thereby darkening the dark values. The midpoint, 50% gray, remains unaffected. Using the two points on my curve, one can see that the y value of point 1 is darker than the x value and the y value of point 2 is lighter than the x value.

Next week, I am going to tackle correcting color balance with curves. Hope you check back.

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